Welcome @AkgunFatih;
Unless you need to use LXC through proxmox and/or bought a barebone server, I would recommend you to directly install Nethserver on your Hetzner instance than use docker to replace the LXC layer.
Since Hetzner already virtualize your instance you will receive poor performance because what you try it like installing proxmox inside proxmox than installing your nethserver inside your proxmox which is inside a proxmox. (But actually they probably use OpenStack).
Already it will be easier for you to manage your nethserver and your “network” than you will also have acceptable performance. On nethserver you will be able to have VM with virtualbox and/or if you prefer KVM it is also possible to install it.
If you still want to keep your scenario than I invite you to read this post first
than ask more questions after 